Thermal interface material and method for making same

ABSTRACT

A thermal interface material includes a curable matrix, a plurality of thermally conductive fibers, and a plurality of the thermally conductive fillers. The thermally conductive fibers and the thermally conductive fillers are embedded in the matrix. The thermally conductive fibers and the thermally conductive fillers are embedded in the matrix. The thermally conductive fillers and the thermally conductive fibers interconnect with each other to cooperatively form a thermally conductive framework in the matrix. A method for making the thermal interface material is also provided.

TECHNICAL FIELD

The invention relates generally to thermal interface materials, and more particularly to a thermal interface material with fillers therein and a method of making the thermal interface material.

BACKGROUND

Many electronic components such as semiconductor chips are becoming progressively smaller with each new product release, while at the same time the heat dissipation requirements of these kinds of components are increasing due to their improved ability to provide more functionality. Commonly, a thermal interface material (hereinafter, TIM) is utilized between an electronic component and a heat sink in order to fill air spaces therebetween and thereby promote efficient heat transfer.

Conventional TIMs are based on thermosetting or thermoplastic polymeric matrices. Nevertheless, the thermal conductivity of conformable polymers is rather low, typically in the range of 0.15 to 0.30 W/mK. To increase the thermal conductivity of the TIM, thermally conductive fillers are generally embedded into the polymeric matrices. The thermal conductivity of these embedded TIMs depends on various factors, such as for example the thermal conductivity of the fillers, and the packing characteristics of the fillers in the polymeric matrix. Packing characteristics include the proportion by weight (or volume) of the fillers in the matrix, the size(s) of the fillers, and the size distribution of the fillers in the matrix.

Generally, a TIM with a high content of fillers has a relatively high thermal conductivity. However, the higher the content of fillers in the TIM, the higher the viscosity of the matrix. In addition, different size distributions of filler particles in a TIM also increase the viscosity of the matrix. If a matrix is highly viscous, the plasticity of the matrix is reduced. This in turn decreases the ability of the TIM to make close contact with a heat transmitting surface such as that of an electronic component and a heat receiving surface such as that of a heat sink. That is, the effectiveness of the TIM is reduced.

Theoretically, the lower the thermal resistance of the TIM, the greater the heat flow from the electronic component to the heat sink. In a typical embedded TIM, the matrix generally occupies spaces between adjacent filler particles. That is, adjacent filler particles do not cooperatively form continuous high heat conduction paths from the surface of the TIM adjoining the electronic component to the opposite surface of the TIM adjoining the heat sink. This means that an overall thermal resistance of the TIM is rather high. That is, the effectiveness of the TIM may be unsatisfactory.

What is needed, therefore, is a TIM that has low thermal resistance, good plasticity, and improved heat conducting efficiency.

What is also needed is a method for making the above-desired TIM.

SUMMARY

In accordance with one embodiment, a thermal interface material includes a matrix, a plurality of thermally conductive fibers, and a plurality of the thermally conductive fillers. The fibers and the fillers are embedded in the matrix. The thermally conductive fillers and the thermally conductive fibers interconnect with each other to cooperatively form a thermally conductive framework in the matrix.

In accordance with another embodiment, a method for making the thermal interface material includes the steps of: interconnecting a plurality of thermally conductive fillers and a plurality of thermally conductive fibers and immersing them in a curable matrix; and curing the matrix, thereby forming the thermal interface material.

Other objects, advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the thermal interface material and the method relating thereto can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present thermal interface material. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the views.

FIG. 1 is a schematic, side view of part of a thermal interface material according to a preferred embodiment of the present invention.

FIG. 2 is a flow chart of an exemplary method for making the thermal interface material of FIG. 1.

FIG. 3 is a side view of the thermal interface material of the preferred embodiment sandwiched between a heat sink and a heat source mounted on a printed circuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present thermal interface material and method relating thereto will now be described in detail below and with reference to the drawings.

Referring to FIG. 1, a thermal interface material (TIM) 10 according to a preferred embodiment of the present invention includes a matrix 12, a plurality of thermally conductive fibers 14, and a plurality of thermally conductive fillers 16. On a large scale, the fibers 14 are generally uniformly distributed and embedded in the matrix 12. On a small scale, the fibers 14 are randomly distributed and embedded in the matrix 12. On a large scale, the fillers 16 are generally uniformly distributed and embedded in the matrix 12. On a small scale, the fillers 16 are randomly distributed and embedded in the matrix 12. Further, the fibers 14 and the fillers 16 are thoroughly mixed with and among each other, at a sufficiently high density such that numerous fibers 14 and fillers 16 are interconnected with one another and cooperatively form a thermally conductive framework in the matrix 12. The TIM 10 advantageously has a thickness in the range from approximately 1 micrometer to approximately 100 micrometers.

Preferably, the fillers 16 each connect with one or more adjacent fibers 14. Thus the fibers 14 serve as thermal paths interconnecting the dispersed fillers 16, whereby the fillers 16 and the fibers 14 cooperatively form a multiplicity of continuous heat conduction paths between a surface of the TIM 10 corresponding to a heat source and another surface of the TIM 10 corresponding to a heat spreader. That is, a thermal resistance between the two surfaces of the TIM 10 is low. In the following description, the two above-mentioned surfaces may be referred to as thermal contact interfaces of the TIM 10.

The matrix 12 is preferably comprised of, for example, a macromolecular material selected from the group consisting of polyvinylacetate, polyvinyl, silicone grease, polyorganosiloxane, polyvinyl chloride, polyol, epoxies, polyester, polyacrylic acid, polypropylene, polyoxymethylene, polyacetal, polyvinyl alcohol, polyolefin, and any suitable combinations thereof. In the illustrated embodiment, the matrix 12 is formed by a crosslinking reaction of vinyl-bearing polydimethylsiloxane, i.e., R¹ _(n) SiO_((4-n)/2), and alkoxysilane, i.e., (R²n(SiOR³)_((4-n)), wherein R¹, R², R³ represent different alkyls or the same alkyl, and n represents the number of respective function groups.

The fibers 14 are preferably comprised of, for example, a thermally conductive material selected from the group consisting of carbon nanotubes, metal fibers, oxide fibers, boron nitride nanotubes, carbon fibers, and any suitable combinations thereof. The metal fibers can be made from thermally conductive metal, for example, silver, gold, copper, nickel, or aluminum. The oxide fibers can be made from thermally conductive oxides, for example, alumina, magnesium oxide, or zinc oxide. The fibers 14 have a diameter of less than 1 micrometer approximately.

In the illustrated embodiment, the fibers 14 are dispersedly distributed and (on a small scale) randomly embedded in the matrix 12. Nevertheless, in alternative embodiments, the fibers 14 can be aligned. Such aligned fibers 14 can, for e.g., be aligned carbon nanotubes, metal fibers, or boron nitride nanotubes. The fibers 14 can be aligned perpendicular to the thermal contact interfaces of the TIM 10. Further, some fibers 14 can be aligned in other orientations. The fillers 16 can be adsorbed on the aligned fibers 14 or embedded between adjacent aligned fibers 14. In this way, the aligned fibers 14 and the fillers 16 cooperatively provide interconnected heat conduction paths in the perpendicular direction and in other directions within the TIM 10. Thereby, the heat conducting efficiency of the TIM 10 is improved. The aligned fibers 14 can be obtained by, for e.g., a chemical vapor deposition method or a plasma-enhanced chemical vapor deposition method.

The fillers 16 are preferably comprised of, for example, a thermally conductive material selected from the group consisting of silver, gold, copper, nickel, aluminum, alumina, zinc oxide, boron nitride, aluminum nitride, graphite, and carbon black. The fillers 16 generally have a grain size of less than 1 micrometer approximately.

Furthermore, a ratio of the fibers 14 to the fillers 16 by weight is advantageously in the range from approximately 1:10 to approximately 1:1. A ratio of a total weight of the fibers 14 plus fillers 16 to a weight of the matrix 12 is in the range from approximately 1:1 to approximately 20:1.

FIG. 2 shows main steps of an exemplary method for making the above-described TIM 10. The exemplary method includes the following steps: providing a plurality of thermally conductive fillers, a plurality of thermally conductive fibers, and a liquid curable material; loosely interconnecting the fillers and the fibers and immersing them in the liquid curable material to form a thermally conductive framework in the liquid curable material; and curing the liquid curable material, thereby forming the TIM 10. In the TIM 10, what was the liquid curable material is the matrix of the TIM 10.

A ratio of the fibers to the fillers by weight is advantageously in the range from approximately 1:10 to approximately 1:1. A ratio of a total weight of the fibers plus fillers to a weight of the matrix is in the range from approximately 1:1 to approximately 20:1. The liquid curable material can be provided in the form of a molten matrix material. Alternatively, the liquid curable material can be provided in the form of a matrix material solution. That is, the solution includes a volatile solvent (for example, alcohol or acetone) having a matrix material dissolved therein.

Briefly, the loosely interconnecting and immersing step preferably includes the steps of: mixing the fillers and the fibers to form a thermally conductive loose framework; and immersing the loose framework into the liquid curable material, thereby positioning the loose framework therein. Alternatively, the loosely interconnecting and immersing step can be performed by directly mixing the fillers and the fibers in the liquid curable material to form a thermally conductive loose framework therein. Each of the above-described mixing processes is preferably performed in a machine; for example, a three roller-mingling machine, a planetary mingling machine, or a grinding machine.

In addition, if the liquid curable material is provided in solution form (see above), a step of evaporating the volatile solvent is further performed prior to the curing of the liquid curable material.

After the curing of the liquid curable material, the fillers, the fibers and the matrix generally form a composite TIM precursor. Preferably, the method further includes a step of grinding the precursor; for example, by using a three roller-mingling machine. Furthermore, since the TIM is generally in the form of a sheet, the ground precursor can be optionally laminated to form a sheet. This can be done, for e.g., by using a rolling press machine.

FIG. 3 illustrates an exemplary application of the TIM 10 when it is in the form of a sheet. In the exemplary application, the TIM 10 is used for dissipating heat generated from a heat source 20. The TIM 10 is generally sandwiched between the heat source 20 (e.g., an electronic component like a central processing unit (CPU)) and a heat spreader 30 (e.g., a heat sink or a heat pipe). In the illustrated embodiment, the heat source 20 is a CPU 20, which is typically mounted on a printed circuit board (PCB) 40. The heat spreader 30 is a heat sink 30, which has a plurality of heat fins 32 extending from a base 34 thereof.

In operation, the CPU 20 generates heat due to power dissipation within its circuitry. The TIM 10 is applied to reduce the thermal resistance of the interface that would otherwise exist between the CPU 20 and the base 34 of the heat sink 30, thus increasing the efficiency of heat transfer from the CPU 20 to the heat sink 30. The heat transferred to the heat sink 30 is radiated into the ambient environment by the fins 32 of the heat sink 30.

During the process of heat conduction by the TIM 10, the fibers 14 and the fillers 16 in the TIM 10 can provide multiple heat conduction paths between the CPU 20 and the heat sink 30, thereby decreasing the thermal resistance between the CPU 20 and the heat sink 30. Because the TIM 10 includes the integrated fibers 14 as well as the fillers 16, the amount of fillers 16 can be reduced. This helps control a viscosity of the matrix 12 without necessarily decreasing the thermal conductivity of the TIM 10. Therefore the plasticity of the matrix 12 and the TIM 10 is good, so that the TIM 10 can make close contact with the CPU 20 and the heat sink 30. Thereby, the heat conducting efficiency of the TIM 10 is improved.

Moreover, because of the structural integrity of the fibers 14 and fillers 16, typically there is little or no “bleeding” from around a periphery of the TIM 10 during operation of the CPU 20.

It is understood that the above-described embodiments and methods are intended to illustrate rather than limit the invention. Variations may be made to the embodiments and methods without departing from the spirit of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention. 

1. A thermal interface material comprising: a matrix; a plurality of thermally conductive fibers embedded in the matrix; and a plurality of the thermally conductive fillers embedded in the matrix, the thermally conductive fillers and the thermally conductive fibers interconnecting with each other to cooperatively form a thermally conductive framework in the matrix.
 2. The thermal interface material as claimed in claim 1, wherein the thermally conductive fillers each interconnect with one or more adjacent thermally conductive fibers.
 3. The thermal interface material as claimed in claim 1, wherein the thermally conductive fillers are comprised of a thermally conductive material selected from the group consisting of silver, gold, copper, nickel, aluminum, alumina, aluminum nitride, boron nitride, zinc oxide, graphite, carbon black, and any combination thereof.
 4. The thermal interface material as claimed in claim 1, wherein the thermally conductive fillers have an average grain size of less than about 1 micrometer.
 5. The thermal interface material as claimed in claim 1, wherein the thermally conductive fibers are comprised of a thermally conductive material selected from the group consisting of carbon nanotubes, metal fibers, oxide fibers, boron nitride nanotubes, carbon fibers, and any combination thereof.
 6. The thermal interface material as claimed in claim 1, wherein the thermally conductive fibers have a diameter of less than about 1 micrometer.
 7. The thermal interface material as claimed in claim 1, wherein a ratio of the fibers to the fillers by weight is in the range from about 1:10 to about 1:1.
 8. The thermal interface material as claimed in claim 1, wherein a ratio of a total weight of the fibers plus fillers to a weight of the matrix is in the range from about 1:1 to about 20:1.
 9. The thermal interface material as claimed in claim 1, wherein the matrix is comprised of a macromolecular material selected from the group consisting of polyvinylacetate, polyvinyl, silicone grease, polyorganosiloxane, polyvinyl chloride, polyol, epoxies, polyester, polyacrylic acid, polypropylene, polyoxymethylene, polyacetal, polyvinyl alcohol, polyolefin, and any combination thereof.
 10. The thermal interface material as claimed in claim 1, wherein the thermal interface material has a thickness in the range from about 1 micrometer to about 100 micrometers.
 11. A method for making a thermal interface material, comprising the steps of: loosely interconnecting a plurality of thermally conductive fillers and a plurality of thermally conductive fibers in a liquid curable material; and curing the liquid curable material, whereby the fillers and the fibers interconnect with each other to cooperatively form a thermally conductive framework in a matrix of the cured material.
 12. The method according to claim 11, wherein the loosely interconnecting step comprises: mixing the fillers and the fibers to form a thermally conductive loose framework; and submerging the loose framework in the liquid curable material.
 13. The method according to claim 11, wherein the loosely interconnecting step is performed by directly mixing the fillers and the fibers in the liquid curable material to form a thermally conductive loose framework in the liquid curable material.
 14. The method according to claim 11, wherein the thermally conductive framework in the matrix defines a thermal interface material precursor, and the method further comprises the step of grinding the precursor using a three roller-mingling machine.
 15. The method according to claim 11, wherein the loosely interconnecting step is performed in a machine selected from the group consisting of a three roller-mingling machine, a planetary mingling machine, and a grinding machine.
 16. The method according to claim 11, wherein the thermally conductive fibers are comprised of a thermally conductive material selected from the group consisting of carbon nanotubes, metal fibers, oxide fibers, boron nitride nanotubes, carbon fibers, and any combination thereof.
 17. A thermal management system comprising: a heat source; a heat spreader; and a thermal interface material between the heat source and the heat spreader, the thermal interface material comprising: a matrix; a plurality of thermally conductive fibers embedded in the matrix; and a plurality of the thermally conductive fillers embedded in the matrix, the thermally conductive fillers and the thermally conductive fibers interconnecting with each other to cooperatively form a thermally conductive framework in the matrix. 